Polymer compositions having improved homogeneity and odour, a method for making them and pipes made thereof

ABSTRACT

The present invention deals with polymer compositions suitable for making pipes. The compositions comprise a multimodal copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms wherein the multimodal ethylene copolymer has a density of from 924 to 960 kg/m 3 , a melt index MFR 5  of from 0.5 to 6.0 g/10 min, a melt index MFR 2  of from 0.1 to 2.0 g/10 min and a shear thinning index SHI 2.7/210  of from 2 to 50. The compositions further have a level of volatile compounds of at most 100 ppm by weight and/or a homogeneity rating of at most 3. In addition the multimodal copolymer comprises:
     (A) from 35 to 60% by weight, based on the combined amount of components (A) and (B), of a low molecular weight ethylene polymer selected from ethylene homopolymer and a copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms and having a weight average molecular weight of from 5000 to 100000 g/mol and a density of from 945 to 975 kg/m 3 ; and   (B) from 40 to 65% by weight, based on the combined amount of components (A) and (B), of a high molecular weight copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms and having a weight average molecular weight of from 100000 to 1000000 g/mol and a density of from 890 to 935 kg/m 3 .

This application, filed under 35 U.S.C. §371, is based on and claimspriority to International Application PCT/EP2009/056303 filed May 25,2009, which claims priority to European Patent Application No.08010009.2, filed on Jun. 2, 2008, the disclosures of which are hereinincorporated by reference in their entireties.

OBJECTIVE OF THE INVENTION

The present invention is directed for polymer compositions for makingpipes. Especially, the present invention is directed for polymercompositions for making pipes having good mechanical properties,improved homogeneity, reduced level of volatiles and which are usefulfor transporting fluids under pressure. In addition the presentinvention is directed to pipes made of the polymer compositions and tomethods of making them.

TECHNICAL BACKGROUND AND PRIOR ART

Pipes made of polyethylene have become popular in transporting water orgas, for instance in houses and in municipal water distribution.Polyethylenes having a high or medium density are frequently used insuch pipes due to their good mechanical properties and ability towithstand pressure. Especially pipes made of multimodal polyethylenehaving a density of from about 947 to 953 kg/m³ have become increasinglypopular. Such pipes and polymer compositions suitable for making themare disclosed, among others, in WO-A-00/01765, WO-A-00/22040,EP-A-739937, EP-A-1141118, EP-A-1041113, EP-A-1330490, EP-A-1328580 andEP-A-1425344. A co-pending European Patent Application No. 06020872.5discloses flexible pressure-resistant pipes made of bimodal polyethyleneand having a density of from 940 to 947 kg/m³.

Such pipes, however, suffer from the disadvantage that the pipes made ofHDPE materials are not flexible enough so that they could be wound to acoil which is preferred in certain applications. Flexible pipes havebeen made from linear low density polyethylene and they are disclosed,among others, in EP-A-1574549. A co-pending European Patent ApplicationNo. 06024952.1 discloses flexible pipes in PE63 category having adensity below 940 kg/m³. However, such pipes often lack the sufficientmechanical properties that are required from pipes used for transportingwater or gas at high pressure. Especially such pipes do not qualify forPE80 or PE100 category.

SUMMARY OF THE INVENTION

The disadvantages of the prior art compositions and pipes are solved bythe present polymer compositions and pipes made of them. Especially, thepolymer compositions are flexible so that the pipes made of them caneasily be bent and coiled. Additionally, the polymer compositions have areduced level of volatile compounds which could cause bad odour.Subsequently the volatile compounds could migrate from the pipe into thewater transported therein causing taste and/or odour problems in water.Furthermore, the polymer compositions have acceptable homogeneitycombined with good mechanical properties and the resulting pipes fulfilthe requirements of PE80 or PE100 classification without having anexcessive amount of inhomogenities, such as white dots.

As seen from one aspect, the present invention provides polymercompositions comprising a multimodal copolymer of ethylene and one ormore alpha-olefins having from 4 to 10 carbon atoms, the multimodalcopolymer having a density from 924 to 960 kg/m³, an MFR₅ of from 0.4 to6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min, an SHI_(2.7/210) from1 to 30, and the composition has a level of volatile compounds of atmost 100 ppm by weight.

As seen from yet another aspect, the present invention provides pipesmade of the polymer compositions comprising a multimodal copolymer ofethylene and one or more alpha-olefins having from 4 to 10 carbon atoms,the multimodal copolymer having a density from 924 to 960 kg/m³, an MFR₅of from 0.4 to 6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min, anSHI_(2.7/210) from 1 to 30, and the composition has a level of volatilecompounds of at most 100 ppm by weight.

As seen from still another aspect, the present invention provides amethod for making pipes wherein the method comprises the steps of:

-   (i) polymerising, in a first polymerisation step in a first    polymerisation zone, in the presence of a single site polymerisation    catalyst, ethylene, hydrogen and optionally one or more    alpha-olefins having 4 to 10 carbon atoms to form the low molecular    weight component (A) having a weight average molecular weight of    from 5000 to 100000 g/mol and a density of from 945 to 977 kg/m³;-   (ii) polymerising, in a second polymerisation step in a second    polymerisation zone, in the presence of a single site polymerisation    catalyst, ethylene, one or more alpha-olefins having 4 to 10 carbon    atoms and optionally hydrogen to form the high molecular weight    component (B) having a weight average molecular weight of from    100000 to 1000000 g/mol and a density of from 890 to 935 kg/m³;    wherein the first polymerisation step and the second polymerisation    step may be conducted in any order and the subsequent step is    conducted in the presence of the polymer produced in the prior step    and the components (A) and (B) are present in the amounts of 30 to    70% and 70 to 30%, respectively, based on the combined amounts of    components (A) and (B), and wherein the multimodal ethylene    copolymer has a density from 924 to 960 kg/m³, an MFR₅ of from 0.4    to 6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min, an    SHI_(2.7/210) from 1 to 30, and the a composition comprising the    multimodal ethylene copolymer has a level of volatile compounds of    at most 100 ppm by weight.

As seen from a further aspect, the present invention provides the use ofthe composition comprising a multimodal copolymer of ethylene and one ormore alpha-olefins having from 4 to 10 carbon atoms, the multimodalcopolymer having a density from 924 to 960 kg/m³, an MFR₅ of from 0.4 to6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min, an SHI_(2.7/210) from1 to 30, and a the composition has a level of volatile compounds of atmost 100 ppm by weight, for making pipes.

DETAILED DESCRIPTION

Below the invention, its preferred embodiments and its advantages aredescribed more in detail.

Multimodal Ethylene Polymer

The multimodal ethylene copolymer is a copolymer of ethylene and atleast one alpha-olefin having from 4 to 10 carbon atoms. It has adensity of from 924 to 960 kg/m³. Additionally it has a melt index MFR₅of from 0.4 to 6.0 g/10 min, preferably from 0.5 to 2.0 g/10 min andmore preferably from 0.6 to 1.4 g/10 min. Further, it typically has amelt index MFR₂ of from 0.1 to 2.0 g/10 min, preferably from 0.2 to 1.0g/10 min and more preferably from 0.2 to 0.45 g/10 min. Additionally ithas a shear thinning index SHI_(2.7/210) of from 1 to 30, preferablyfrom 2 to 20 and more preferably from 3 to 15.

The multimodal ethylene copolymer preferably has a weight averagemolecular weight of from 75000 g/mol to 250000 g/mol, more preferablyfrom 100000 g/mol to 250000 g/mol and in particular from 120000 g/mol to220000 g/mol. Additionally, it preferably has a number average molecularweight of 15000 g/mol to 40000 g/mol, and more preferably 18000 to 30000g/mol. It furthermore preferably has a ratio Mw/Mn of from 4 to 15, morepreferably from 4 to 10.

Preferably the multimodal ethylene copolymer comprises a low molecularweight ethylene polymer component (A) and a high molecular weightethylene copolymer component (B). Especially, the composition preferablycontains from 30 to 70% the low molecular weight polymer (A) and morepreferably from 35 to 50%. In addition, the composition preferablycontains from 70 to 30% by weight of the copolymer (B) and morepreferably from 65 to 50%. The percentage figures are based on thecombined weight of components (A) and (B). The components (A) and (B)are explained more in detail below.

The low molecular weight polymer component (A) is an ethylenehomopolymer or a copolymer of ethylene and one or more alpha-olefinshaving from 4 to 10 carbon atoms. It preferably has a weight averagemolecular weight Mw of from 5000 to 100000, more preferably from 10000to 100000 g/mol, even more preferably from 15000 to 80000 g/mol and inparticular from 15000 to 50000 g/mol. Preferably it has a melt indexMFR₂ of from 20 to 1500 g/10 min. Moreover, it preferably has a narrowmolecular weight distribution having a ratio of the weight averagemolecular weight to the number average molecular weight of from 2 to 5,more preferably from 2 to 4 and in particular from 2 to 3.5.Furthermore, it preferably has a density of from 945 to 977 kg/m³.Especially preferably the low molecular weight ethylene polymer (A) isan ethylene homopolymer.

The high molecular weight polymer component (B) is a copolymer ofethylene and one or more alpha-olefins having from 4 to 10 carbon atoms.It preferably has a weight average molecular weight Mw of from 100000 to1000000 g/mol and more preferably from 150000 to 500000 g/mol.Preferably it has a melt index MFR₂ of from 0.01 to 0.3 g/10 min.Moreover, it preferably has a narrow molecular weight distributionhaving a ratio of the weight average molecular weight to the numberaverage molecular weight of from 2 to 3.5. Furthermore, it preferablyhas a density of from 890 to 935 kg/m³, more preferably from 900 to 929kg/m³.

By ethylene homopolymer is meant a polymer which substantially consistsof ethylene units. As the process streams may have small amount of otherpolymeriseable species as impurities the homopolymer may contain a smallamount of units other than ethylene. The content of such units should belower than 0.2% by mole, preferably less than 0.1% by mole.

By copolymer of ethylene and one or more alpha-olefins having from 4 to10 carbon atoms is meant a copolymer which has a majority of ethyleneunits and substantially consists of units derived from ethylene andalpha-olefins having from 4 to 10 carbon atoms. As the process streamsmay have small amount of other polymeriseable species as impurities thecopolymer may contain a small amount of units other than ethylene andalpha-olefins having from 4 to 10 carbon atoms. The content of suchunits should be lower than 0.2% by mole, preferably less than 0.1% bymole.

The low molecular weight polymer component (A) and the high molecularweight polymer component (B) can also be blends of two or more differentpolymer fractions provided that each fraction, as well as the blend,meets the requirements given above for the specific component.

The multimodal ethylene copolymer may also contain minor amount of otherpolymer, such as prepolymer. The amount of such polymers should notexceed 5%, preferably not 2% by weight of the multimodal ethylenecopolymer.

According to one embodiment of the invention the multimodal ethylenecopolymer has a melt index MFR₅ of 0.5 to 2.0 g/10 min, preferably from0.6 to 1.4 g/10 min. It has a density of from 925 to 935 kg/m³.Furthermore, it has a melt index MFR₂ of 0.1 to 1.0 g/10 min preferablyfrom 0.2 to 0.45 g/10 min. It also has a shear thinning indexSHI_(2.7/210) of from 1 to 30, preferably from 5 to 30.

According to another embodiment of the invention the multimodal ethylenecopolymer has a melt index MFR₅ of 1.0 to 6.0 g/10 min, preferably from1.4 to 6.0 g/10 min. It has a density of from 925 to 935 kg/m³.Furthermore, it has a melt index MFR₂ of 0.4 to 2.0 g/10 min, preferablyfrom 0.5 to 2.0 g/10 min. It also has a shear thinning indexSHI_(2.7/210) of from 2 to 30, preferably from 3 to 15.

Polymerisation Process

The multimodal ethylene copolymer is typically produced in a multistagepolymerisation process in the presence of a single site catalyst.

In the multistage polymerisation process ethylene and alpha-olefinshaving from 4 to 10 carbon atoms are polymerised in a process comprisingat least two polymerisation stages. Each polymerisation stage may beconducted in a separate reactor but they may also be conducted in atleast two distinct polymerisation zones in one reactor. Preferably, themultistage polymerisation process is conducted in two cascadedpolymerisation stages.

Catalyst

The polymerisation is typically conducted in the presence of a singlesite polymerisation catalyst. Preferably the single site catalyst is ametallocene catalyst. Such catalysts comprise a transition metalcompound which contains a cyclopentadienyl, indenyl or fluorenyl ligand.Preferably the catalyst contains two cyclopentadienyl, indenyl orfluorenyl ligands, which may be bridged by a group preferably containingsilicon and/or carbon atom(s). Further, the ligands may havesubstituents, such as alkyl groups, aryl groups, arylalkyl groups,alkylaryl groups, silyl groups, siloxy groups, alkoxy groups and like.Suitable metallocene compounds are known in the art and are disclosed,among others, in WO-A-97128170, WO-A-98/32776, WO-A-99/61489,WO-A-031010208, WO-A-031051934, WO-A-031051514, WO-A-20041085499,EP-A-1752462 and EP-A-1739103.

Especially, the metallocene compound must be capable of producingpolyethylene having sufficiently high molecular weight. Especially ithas been found that metallocene compounds having hafnium as thetransition metal atom or metallocene compounds comprising an indenyl ortetrahydroindenyl type ligand often have the desired characteristics.

One example of suitable metallocene compounds is the group ofmetallocene compounds having zirconium, titanium or hafnium as thetransition metal and one or more ligands having indenyl structurebearing a siloxy substituent, such as[ethylenebis(3,7-di(tri-isopropylsiloxy)inden-1-yl)]zirconium dichloride(both rac and meso),[ethylenebis(4,7-di(tri-isopropylsiloxy)inden-1-yl)]zirconium dichloride(both rac and meso),[ethylenebis(5-tert-butyldimethylsiloxy)inden-1-yl)]zirconium dichloride(both rac and meso), bis(5-tert-butyldimethylsiloxy)inden-1-yl)zirconiumdichloride,[dimethylsilylenenebis(5-tert-butyldimethylsiloxy)inden-1-yl)]zirconiumdichloride (both rac and meso),(N-tert-butylamido)(dimethyl)(η⁵-inden-4-yloxy)silanetitanium dichlorideand [ethylenebis(2-(tert-butydimethylsiloxy)inden-1-yl)]zirconiumdichloride (both rac and meso).

Another example is the group of metallocene compounds having hafnium asthe transition metal atom and bearing a cyclopentadienyl type ligand,such as bis(n-butylcyclopentadienyl)hafnium dichloride,bis(n-butylcyclopentadienyl) dibenzylhafnium,dimethylsilylenenebis(n-butylcyclopentadienyl)hafnium dichloride (bothrac and meso) and bis[1,2,4-tri(ethyl)cyclopentadienyl]hafniumdichloride.

Still another example is the group of metallocene compounds bearing atetrahydroindenyl ligand such as bis(4,5,6,7-tetrahydroindenyl)zirconiumdichloride, bis(4,5,6,7-tetrahydroindenyl)hafnium dichloride,ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride,dimethylsilylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride.

The single site catalyst typically also comprises an activator.Generally used activators are alumoxane compounds, such asmethylalumoxane (MAO), tetraisobutylalumoxane (TIBAO) orhexaisobutylalumoxane (HIBAO). Also boron activators, such as thosedisclosed in US-A-2007/049711 may be used. The activators mentionedabove may be used alone or they may be combined with, for instance,aluminium alkyls, such as triethylaluminium or tri-isobutylaluminium.

The catalyst may be supported. The support may be any particulatesupport, including inorganic oxide support, such as silica, alumina ortitania, or polymeric support, such as polymer-comprising styrene ordivinylbenzene. When a supported catalyst is used the catalyst needs tobe prepared so that the activity of the catalyst does not suffer. Thenthe catalyst residues remaining in the product do not have negativeimpact on the taste and odour properties of the final polymer and thehomogeneity of the polymer is not negatively affected.

The catalyst may also comprise the metallocene compound on solidifiedalumoxane or it may be a solid catalyst prepared according to emulsionsolidification technology. Such catalysts are disclosed, among others,in EP-A-1539775 or WO-A-03/051934. It has surprisingly been found thatwhen such catalyst is used the resulting multimodal polymer has improvedhomogeneity as indicated by reduced number and site of white dots sothat the polymer composition has a low homogeneity rating according toISO 18553 and improved taste and/or odour properties.

According to an especially preferred embodiment the catalyst comprisesan organometallic compound of a transition metal of Group 3 to 10 of thePeriodic Table (IUPAC), or of an actinide or lanthanide, in the form ofsolid catalyst particles and is prepared by a process comprising thefollowing steps:

-   -   preparing a solution of one or more catalyst components;    -   dispersing said solution in a solvent immiscible therewith to        form an emulsion in which one or more catalyst components are        present in the droplets of the dispersed phase; and    -   solidifying said dispersed phase to convert said droplets to        solid particles and optionally recovering said particles to        obtain said catalyst.        Polymerisation

The multimodal ethylene copolymer may be produced in any suitablepolymerisation process known in the art. Into the polymerisation zone isalso introduced ethylene, optionally an inert diluent, and optionallyhydrogen and/or comonomer. The low molecular weight ethylene polymercomponent is produced in a first polymerisation zone and the highmolecular weight ethylene copolymer component is produced in a secondpolymerisation zone. The first polymerisation zone and the secondpolymerization zone may be connected in any order, i.e. the firstpolymerisation zone may precede the second polymerisation zone, or thesecond polymerisation zone may precede the first polymerisation zone or,alternatively, polymerisation zones may be connected in parallel.However, it is preferred to operate the polymerisation zones in cascadedmode. The polymerisation zones may operate in slurry, solution, or gasphase conditions or their combinations. Suitable reactor configurationsare disclosed, among others, in WO-A-92112182, EP-A-369436, EP-A-503791,EP-A-881237 and WO-A-96/18662. Examples of processes where thepolymerisation zones are arranged within one reactor system aredisclosed in WO-A-99/03902, EP-A-782587 and EP-A-1633466.

It is often preferred to remove the reactants of the precedingpolymerisation stage from the polymer before introducing it into thesubsequent polymerisation stage. This is preferably done whentransferring the polymer from one polymerisation stage to another.Suitable methods are disclosed, among others, in EP-A-1415999 andWO-A-00/26258.

The polymerisation in the polymerisation zone may be conducted inslurry. Then the polymer particles formed in the polymerisation,together with the catalyst fragmented and dispersed within theparticles, are suspended in the fluid hydrocarbon. The slurry isagitated to enable the transfer of reactants from the fluid into theparticles.

The polymerisation usually takes place in an inert diluent, typically ahydrocarbon diluent such as methane, ethane, propane, n-butane,isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures.Preferably the diluent is a low-boiling hydrocarbon having from 1 to 4carbon atoms or a mixture of such hydrocarbons. An especially preferreddiluent is propane, possibly containing minor amount of methane, ethaneand/or butane.

The ethylene content in the fluid phase of the slurry may be from 2 toabout 50% by mole, preferably from about 3 to about 20% by mole and inparticular from about 5 to about 15% by mole. The benefit of having ahigh ethylene concentration is that the productivity of the catalyst isincreased but the drawback is that more ethylene then needs to berecycled than if the concentration was lower.

The temperature in the slurry polymerisation is typically from 50 to115° C., preferably from 60 to 110° C. and in particular from 70 to 100°C. The pressure is from 1 to 150 bar, preferably from 10 to 100 bar.

The slurry polymerisation may be conducted in any known reactor used forslurry polymerisation. Such reactors include a continuous stirred tankreactor and a loop reactor. It is especially preferred to conduct thepolymerisation in loop reactor. In such reactors the slurry iscirculated with a high velocity along a closed pipe by using acirculation pump. Loop reactors are generally known in the art andexamples are given, for instance, in U.S. Pat. No. 4,582,816, U.S. Pat.No. 3,405,109, U.S. Pat. No. 3,324,093, EP-A-479186 and U.S. Pat. No.5,391,654.

It is sometimes advantageous to conduct the slurry polymerisation abovethe critical temperature and pressure of the fluid mixture. Suchoperation is described in U.S. Pat. No. 5,391,654.

In such operation the temperature is typically from 85 to 110° C.,preferably from 90 to 105° C. and the pressure is from 40 to 150 bar,preferably from 50 to 100 bar.

The slurry may be withdrawn from the reactor either continuously orintermittently. A preferred way of intermittent withdrawal is the use ofsettling legs where slurry is allowed to concentrate before withdrawinga batch of the concentrated slurry from the reactor. The use of settlinglegs is disclosed, among others, in U.S. Pat. No. 3,374,211, U.S. Pat.No. 3,242,150 and EP-A-1310295. Continuous withdrawal is disclosed,among others, in EP-A-891990, EP-A-1415999, EP-A-1591460 andWO-A-2007/025640. The continuous withdrawal is advantageously combinedwith a suitable concentration method, as disclosed in EP-A-1310295 andEP-A-1591460.

If the low molecular weight ethylene polymer is produced in slurrypolymerisation stage then hydrogen is added to the slurry reactor sothat the molar ratio of hydrogen to ethylene in the reaction phase isfrom 0.1 to 1.0 mol/kmol, and preferably from 0.2 to 0.7 mol/kmol.Comonomer may then also be introduced into the slurry polymerisationstage so that the molar ratio of comonomer to ethylene in the reactionphase does not exceed 150 mol/kmol, and preferably not 50 mol/kmol.Especially preferably no comonomer is introduced into the slurrypolymerisation stage.

If the high molecular weight ethylene polymer is produced in slurrypolymerisation stage then hydrogen is added to the slurry reactor sothat the molar ratio of hydrogen to ethylene in the reaction phase is atmost 0.1 mol/kmol, preferably from 0.01 to 0.07 mol/kmol. Especiallypreferably, no hydrogen is introduced into the slurry polymerisationstage. Comonomer is introduced into the slurry polymerisation stage sothat the molar ratio of comonomer to ethylene is from 50 to 200mol/kmol, preferably from 70 to 120 mol/kmol.

The polymerisation may also be conducted in gas phase. In a fluidisedbed gas phase reactor an olefin is polymerised in the presence of apolymerisation catalyst in an upwards moving gas stream. The reactortypically contains a fluidised bed comprising the growing polymerparticles containing the active catalyst located above a fluidisationgrid.

The polymer bed is fluidised with the help of the fluidisation gascomprising the olefin monomer, eventual comonomer(s), eventual chaingrowth controllers or chain transfer agents, such as hydrogen, andeventual inter gas. The fluidisation gas is introduced into an inletchamber at the bottom of the reactor. To make sure that the gas flow isuniformly distributed over the cross-sectional surface area of the inletchamber the inlet pipe may be equipped with a flow dividing element asknown in the art, e.g. U.S. Pat. No. 4,933,149 and EP-A-684871.

From the inlet chamber the gas flow is passed upwards through afluidisation grid into the fluidised bed. The purpose of thefluidisation grid is to divide the gas flow evenly through thecross-sectional area of the bed. Sometimes the fluidisation grid may bearranged to establish a gas stream to sweep along the reactor walls, asdisclosed in WO-A-2005/087361. Other types of fluidisation grids aredisclosed, among others, in U.S. Pat. No. 4,578,879, EP-A-600414 andEP-A-721798. An overview is given in Geldart and Bayens: The Design ofDistributors for Gas-fluidized Beds, Powder Technology, Vol. 42, 1985.

The fluidisation gas passes through the fluidised bed. The superficialvelocity of the fluidisation gas must be higher that minimumfluidisation velocity of the particles contained in the fluidised bed,as otherwise no fluidisation would occur. On the other hand, thevelocity of the gas should be lower than the onset velocity of pneumatictransport, as otherwise the whole bed would be entrained with thefluidisation gas. The minimum fluidisation velocity and the onsetvelocity of pneumatic transport can be calculated when the particlecharacteristics are know by using common engineering practise. Anoverview is given, among others in Geldart: Gas Fluidization Technology,J. Wiley & Sons, 1986.

When the fluidisation gas is contacted with the bed containing theactive catalyst the reactive components of the gas, such as monomers andchain transfer agents, react in the presence of the catalyst to producethe polymer product. At the same time the gas is heated by the reactionheat.

The unreacted fluidisation gas is removed from the top of the reactorand cooled in a heat exchanger to remove the heat of reaction. The gasis cooled to a temperature which is lower than that of the bed toprevent the bed from heating because of the reaction. It is possible tocool the gas to a temperature where a part of it condenses. When theliquid droplets enter the reaction zone they are vaporised. Thevaporisation heat then contributes to the removal of the reaction heat.This kind of operation is called condensed mode and variations of it aredisclosed, among others, in WO-A-20071025640, U.S. Pat. No. 4,543,399,EP-A-699213 and WO-A-94/25495. It is also possible to add condensingagents into the recycle gas stream, as disclosed in EP-A-696293. Thecondensing agents are non-polymerisable components, such as n-pentane,isopentane, n-butane or isobutane, which are at least partiallycondensed in the cooler.

The gas is then compressed and recycled into the inlet chamber of thereactor. Prior to the entry into the reactor fresh reactants areintroduced into the fluidisation gas stream to compensate for the lossescaused by the reaction and product withdrawal. It is generally known toanalyse the composition of the fluidisation gas and introduce the gascomponents to keep the composition constant. The actual composition isdetermined by the desired properties of the product and the catalystused in the polymerisation.

The catalyst may be introduced into the reactor in various ways, eithercontinuously or intermittently. Among others, WO-A-01/05845 andEP-A-499759 disclose such methods. Where the gas phase reactor is a partof a reactor cascade the catalyst is usually dispersed within thepolymer particles from the preceding polymerisation stage. The polymerparticles may be introduced into the gas phase reactor as disclosed inEP-A-1415999 and WO-A-00/26258.

The polymeric product may be withdrawn from the gas phase reactor eithercontinuously or intermittently. Combinations of these methods may alsobe used. Continuous withdrawal is disclosed, among others, inWO-A-00129452. Intermittent withdrawal is disclosed, among others, inU.S. Pat. No. 4,621,952, EP-A-188125, EP-A-250169 and EP-A-579426.

The top part of the gas phase reactor may include a so calleddisengagement zone. In such a zone the diameter of the reactor isincreased to reduce the gas velocity and allow the particles that arecarried from the bed with the fluidisation gas to settle back to thebed.

The bed level may be observed by different techniques known in the art.For instance, the pressure difference between the bottom of the reactorand a specific height of the bed may be recorded over the whole lengthof the reactor and the bed level may be calculated based on the pressuredifference values. Such a calculation yields a time-averaged level. Itis also possible to use ultrasonic sensors or radioactive sensors. Withthese methods instantaneous levels may be obtained, which of course maythen be averaged over time to obtain time-averaged bed level.

Also antistatic agent(s) may be introduced into the gas phase reactor ifneeded. Suitable antistatic agents and methods to use them aredisclosed, among others, in U.S. Pat. Nos. 5,026,795, 4,803,251,4,532,311, 4,855,370 and EP-A-560035. They are usually polar compoundsand include, among others, water, ketones, aldehydes and alcohols.

The reactor may also include a mechanical agitator to further facilitatemixing within the fluidised bed. An example of suitable agitator designis given in EP-A-707513.

If the low molecular weight ethylene polymer is produced in gas phasepolymerisation stage then hydrogen is added to the gas phase reactor sothat the molar ratio of hydrogen to ethylene is from 0.5 to 1.5mol/kmol, and preferably from 0.7 to 1.3 mol/kmol. Comonomer may thenalso be introduced into the gas phase polymerisation stage so that themolar ratio of comonomer to ethylene does not exceed 20 mol/kmol, andpreferably not 15 mol/kmol. Especially preferably no comonomer isintroduced into the gas phase polymerisation stage.

If the high molecular weight ethylene polymer is produced in gas phasepolymerisation stage then hydrogen is added to the gas phase reactor sothat the molar ratio of hydrogen to ethylene is at most 0.4 mol/kmol,preferably at most 0.3 mol/kmol. Especially preferably, no hydrogen isintroduced into the gas phase polymerisation stage. Comonomer isintroduced into the gas phase polymerisation stage so that the molarratio of comonomer to ethylene is from 5 to 50 mol/mol.

Powder Treatment

When the powder is withdrawn from the polymerisation section it isdegassed and mixed with the desired additives. Degassing is preferablyconducted by purging the polymer with gas at an elevated temperature.

One preferred method of purging the polymer is to pass a continuousstream of polymer powder through a vessel into which a gas stream issimultaneously passed. The gas stream may be either counter-current orco-current with the polymer stream, preferably counter-current. Theresidence time of the polymer is such a vessel may be from 10 minutes to5 hours, preferably from about 30 minutes to about 2 hours. The gas usedin purging may be ethylene, nitrogen, steam, air etc. Particularly goodresults have been obtained by using nitrogen as purging gas, whichpreferably contains a small amount of steam, such as from 100 ppm to 5%by weight, preferably from 100 ppm to 1%.

The temperature at which the polymer and gas are contacted may rangefrom 30 to 100° C., preferably from 40 to 90° C. The temperature must belower than the melting temperature of the multimodal ethylene copolymer.On the other hand, the temperature must be sufficiently high to make thevolatile compounds to evaporate and migrate from the polymer into thegas stream.

Suitable gas stream in the method described above is from 0.01 to 5 tongas per one ton of polymer.

Other suitable treatment methods may also be used. Thus, a batch ofpolymer may be purged in a vessel under a gas stream for a suitableperiod of time.

Polymer Composition

In addition to the multimodal ethylene copolymer the polymer compositioncomprises additives, fillers and adjuvants known in the art. It may alsocontain additional polymers, such as carrier polymers of the additivemasterbatches. Preferably the polymer composition comprises at least 50%by weight of the multimodal ethylene copolymer, preferably from 80 to100% by weight and more preferably from 85 to 100% by weight, based onthe total weight of the composition.

Suitable antioxidants and stabilizers are, for instance, stericallyhindered phenols, phosphates or phosphonites, sulphur containingantioxidants, alkyl radical scavengers, aromatic amines, hindered aminestabilizers and the blends containing compounds from two or more of theabove-mentioned groups.

Examples of sterically hindered phenols are, among others,2,6-di-tert-butyl-4-methyl phenol (sold, e.g., by Degussa under a tradename of Ionol CP),pentaerythrityl-tetrakis(3-(3′,5′-di-tert.butyl-4-hydroxyphenyl)-propionate(sold, e.g., by Ciba Specialty Chemicals under the trade name of Irganox1010) octadecyl-3-3(3′5′-di-tert-butyl-4′-hydroxyphenyl)propionate(sold, e.g., by Ciba Specialty Chemicals under the trade name of Irganox1076) and 2,5,7,8-tetramethyl-2(4′,8′,12′-trimethyltridecyl)chroman-6-ol(sold, e.g., by BASF under the trade name of Alpha-Tocopherol).

Examples of phosphates and phosphonites are tris (2,4-di-t-butylphenyl)phosphite (sold, e.g., by Ciba Specialty Chemicals under the trade nameof Irgafos 168),tetrakis-(2,4-di-t-butylphenyl)-4,4′-biphenylen-di-phosphonite (sold,e.g., by Ciba Specialty Chemicals under the trade name of Irgafos P-EPQ)and tris-(nonylphenyl)phosphate (sold, e.g., by Dover Chemical under thetrade name of Doverphos HiPure 4)

Examples of sulphur-containing antioxidants are dilaurylthiodipropionate(sold, e.g., by Ciba Specialty Chemicals under the trade name of IrganoxPS 800), and distearylthiodipropionate (sold, e.g., by Chemtura underthe trade name of Lowinox DSTDB).

Examples of nitrogen-containing antioxidants are4,4′-bis(1,1′-dimethylbenzyl)diphenylamine (sold, e.g., by Chemturaunder the trade name of Naugard 445), polymer of2,2,4-trimethyl-1,2-dihydroquinoline (sold, e.g., by Chemtura under thetrade name of Naugard EL-17), p-(p-toluene-sulfonylamido)-diphenylamine(sold, e.g., by Chemtura under the trade name of Naugard SA) andN,N′-diphenyl-p-phenylene-diamine (sold, e.g., by Chemtura under thetrade name of Naugard J).

Commercially available blends of antioxidants and process stabilizersare also available, such as Irganox B225, Irganox 8215 and Irganox B561marketed by Ciba-Geigy.

Suitable acid scavengers are, for instance, metal stearates, such ascalcium stearate and zinc stearate. They are used in amounts generallyknown in the art, typically from 500 ppm to 10000 ppm and preferablyfrom 500 to 5000 ppm.

Carbon black is a generally used pigment, which also acts as anUV-screener. Typically carbon black is used in an amount of from 0.5 to5% by weight, preferably from 1.5 to 3.0% by weight. Preferably thecarbon black is added as a masterbatch where it is premixed with apolymer, preferably high density polyethylene (HDPE), in a specificamount. Suitable masterbatches are, among others, HD4394, sold by CabotCorporation, and PPM1805 by Poly Plast Muller. Also titanium oxide maybe used as an UV-screener.

The polymer composition comprising the multimodal ethylene copolymer haspreferably a low level of volatile compounds. Thus, the level ofvolatile compounds measured from the pellets made of the composition isat most 100 ppm by weight, preferably at most 75 ppm by weight and morepreferably at most 50 ppm by weight. Typical values measured from thepelletised material may be from 1 to 30 ppm by weight.

Additionally the composition comprising the multimodal ethylenecopolymer preferably has acceptable homogeneity. Thus, it preferably hasa rating according to ISO 18553 of less than 6, more preferably of atmost 5 and in particular of at most 4.5. As the person skilled in theart knows, the minimum rating is 0 for a completely homogeneousmaterial.

Homogenisation and Pelletisation

The composition comprising the multimodal ethylene copolymer ishomogenised and pelletised using a method known in the art. Preferably,a twin screw extruder is used. Such extruders are known in the art andthey can be divided in co-rotating twin screw extruders, as disclosed inWO-A-98/15591, and counter-rotating twin screw extruders, as disclosedin EP-A-1600276. In the co-rotating twin screw extruder the screwsrotate in the same direction whereas in the counter-rotating extruderthey rotate in opposite directions. An overview is given, for example,in Rauwendaal: Polymer Extrusion (Hanser, 1986), chapters 10.3 to 10.5,pages 460 to 489. Especially preferably a counter-rotating twin screwextruder is used.

To ensure sufficient homogenisation of the polymer composition duringthe extrusion the specific energy input must be on a sufficiently highlevel. On the other hand, it must not be excessive, as otherwisedegradation of polymer would occur. Also the additives could partlydegrade due to too high energy input, and the degradation products ofthe polymer and the additives could cause offensive odour and/or tastein the polymer. The required SEI level depends somewhat on the screwconfiguration and design. Suitable levels of specific energy input (SEI)are from 200 to 300 kWh/ton, preferably from 210 to 290 kWh/ton.Especially good results have been obtained when the SEI is within therange disclosed above and a counter-rotating twin screw extruder havinga screw design according to EP-A-1600276 is used.

Pipe and Pipe Manufacture

Pipes according to the present invention are produced according to themethods known in the art from the polymer composition as describedabove. Thus, according to one preferred method the polymer compositionis extruded through an annular die to a desired internal diameter, afterwhich the polymer composition is cooled.

The pipe extruder preferably operates at a relatively low temperatureand therefore excessive heat build-up should be avoided. Extrudershaving a high length to diameter ratio L/D more than 15, preferably ofat least 20 and in particular of at least 25 are preferred. The modernextruders typically have an L/D ratio of from about 30 to 35.

The polymer melt is extruded through an annular die, which may bearranged either as end-fed or side-fed configuration. The side-fed diesare often mounted with their axis parallel to that of the extruder,requiring a right-angle turn in the connection to the extruder. Theadvantage of side-fed dies is that the mandrel can be extended throughthe die and this allows, for instance, easy access for cooling waterpiping to the mandrel.

After the plastic melt leaves the die it is calibrated to the correctdiameter. In one method the extrudate is directed into a metal tube(calibration sleeve). The inside of the extrudate is pressurised so thatthe plastic is pressed against the wall of the tube. The tube is cooledby using a jacket or by passing cold water over it.

According to another method a water-cooled extension is attached to theend of the die mandrel. The extension is thermally insulated from thedie mandrel and is cooled by water circulated through the die mandrel.The extrudate is drawn over the mandrel which determines the shape ofthe pipe and holds it in shape during cooling. Cold water is flowed overthe outside pipe surface for cooling.

According to still another method the extrudate leaving the die isdirected into a tube having perforated section in the centre. A slightvacuum is drawn through the perforation to hold the pipe hold the pipeagainst the walls of the sizing chamber.

After the sizing the pipe is cooled, typically in a water bath having alength of about 5 meters or more.

The pipes according to the present invention preferably fulfil therequirements of PESO standard as defined in EN 12201 and EN 1555,evaluated according to ISO 9080, or alternatively ISO 4427.

The pipes according to the present invention are especially suited fortransporting water or gas under pressure. Especially, they are suitablefor transporting drinking water. No compounds producing offensive odouror taste into the water are migrated from the pipe into the water.

EXAMPLES

Methods

Melt Index

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the melt viscosity ofthe polymer. The MFR is determined at 190° C. for PE. The load underwhich the melt flow rate is determined is usually indicated as asubscript, for instance MFR₂ is measured under 2.16 kg load (conditionD), MFR₅ is measured under 5 kg load (condition T) or MFR₂₁ is measuredunder 21.6 kg load (condition G).

The quantity FRR (flow rate ratio) is an indication of molecular weightdistribution and denotes the ratio of flow rates at different loads.Thus, FRR_(21/2) denotes the value of MFR₂₁/MFR₂.

Density

Density of the polymer was measured according to ISO 1183/1872-2B.

For the purpose of this invention the density of the blend can becalculated from the densities of the components according to:

$\rho_{b} = {\sum\limits_{i}\;{w_{i} \cdot \rho_{i}}}$where ρ_(b) is the density of the blend,

-   -   w_(i) is the weight fraction of component “i” in the blend and    -   ρ_(i) is the density of the component “i”.        Molecular Weight

Mw, Mn and MWD are measured by Gel Permeation Chromatography (GPC)according to the following method:

The weight average molecular weight Mw and the molecular weightdistribution (MWD=Mw/Mn wherein Mn is the number average molecularweight and Mw is the weight average molecular weight) is measuredaccording to ISO 16014-4:2003 and ASTM D 6474-99. A Waters GPCV2000instrument, equipped with refractive index detector and onlineviscosimeter was used with 2×GMHXL-HT and 1×G7000HXL-HT TSK-gel columnsfrom Tosoh Bioscience and 1,2,4-trichlorobenzene (TCB, stabilized with250 mg/L 2,6-Di tert-butyl-4-methyl-phenol) as solvent at 140° C. and ata constant flow rate of 1 mL/min. 209.5 μL of sample solution wereinjected per analysis. The column set was calibrated using universalcalibration (according to ISO 16014-2:2003) with at least 15 narrow MWDpolystyrene (PS) standards in the range of 1 kg/mol to 12 000 kg/mol.Mark Houwink constants were used as given in ASTM D 6474-99. All sampleswere prepared by dissolving 0.5-4.0 mg of polymer in 4 mL (at 140° C.)of stabilized TCB (same as mobile phase) and keeping for max. 3 hours ata maximum temperature of 160° C. with continuous gentle shaking priorsampling in into the GPC instrument.

As it is known in the art, the weight average molecular weight of ablend can be calculated if the molecular weights of its components areknown according to:

${Mw}_{b} = {\sum\limits_{i}\;{w_{i} \cdot {Mw}_{i}}}$where Mw_(b) is the weight average molecular weight of the blend, w_(i)is the weight fraction of component “i” in the blend and Mw_(i) is theweight average molecular weight of the component “i”.

The number average molecular weight can be calculated using the mixingrule:

$\frac{1}{{Mn}_{b}} = {\sum\limits_{i}\;\frac{w_{i}}{{Mn}_{i}}}$where Mn_(b) is the weight average molecular weight of the blend, w_(i)is the weight fraction of component “i” in the blend and Mn_(i) is theweight average molecular weight of the component “i”.Homogeneity

The homogeneity of the polymer samples containing carbon black wasdetermined by using optical microscope according to the method ISO 18553as follows.

A predetermined amount of the polymer of the Examples was mixed 5.75% ofHE0880 carbon black masterbatch in a Brabender 350 E mixer with a Rollerelement at 190° C. temperature for 10 minutes. The screw speed was 20RPM. Materials were then transferred to a compression moulding devicefor making about 3 mm thick plates (about 5×5 cm). Moulding conditions:200 C during 10 minutes at low pressure and for 5 minutes at 114 bar andcooling at 15 C/min. Pellets of about 6 mm diameter were punched outfrom plates and then taken to homogeneity assessment.

A sample of the composition (containing the pigment) was obtained and atleast 6 microtome cuts were made from different parts of the sample.Each cut had a thickness of about 12 μm (if carbon black was used; forother pigments the thickness may be from 15 to 35 μm). The diameter ofthe microtome cuts is from 3 to 5 mm. The cuts are evaluated at amagnification of 100. The diameters of the inhomogeneities(non-pigmented areas or “white spots”) are determined and a rating isgiven according to the rating scheme of ISO 18553. The lower is therating the more homogeneous is the material.

Gel Level

The gels were determined from 0.3 mm sheet as follows.

A predetermined amount of the polymer of the Examples was mixed in aBrabender 350 E mixer with a Roller element at 190° C. temperature for10 minutes. The screw speed was 20 RPM. Materials were then transferredto a compression moulding device for making about 0.3 mm thick sheets(about 20×20 cm). Moulding conditions: 200 C during 5 minutes at lowpressure and for 5 minutes at 114 bar and cooling at 15 C/min.

The plates were inspected for gels over a glass table illuminated frombelow. The table is 0.5×0.3 m of size and equipped with threefluorescent lamps, each of them 15 W and with a warm white light. Thelamps were covered with an opaque glass plate. The gels were dividedinto following classes according to size:

-   Class 1: More than or equal to 0.7 mm-   Class 2: 0.4-0.7 mm    Volatile Content

The total emission of the polymers was determined by using multiple headspace extraction according to method as described below. If notmentioned otherwise all reported data refer to this method.

The method for measuring volatile components is carried out as follows:

The volatile components as described above were determined by using agas chromatograph and a headspace method. The equipment was a HewlettPackard gas chromatograph with a 25 m×0.32 mm×2.5 μm(length×diameter×size of packing material) non-polar column filled withDB-1 (100% dimethyl polysiloxane). A flame ionisation detector was usedwith hydrogen as a fuel gas. Helium at 10 psi was used as a carrier gaswith a flow rate of 3 ml/min. After the injection of the sample the oventemperature was maintained at 50° C. for 3 minutes, after which it wasincreased at a rate of 12° C./min until it reached 200° C. Then the ovenwas maintained at that temperature for 4 minutes, after which theanalysis was completed.

The calibration was carried out as follows: At least three andpreferably from five to ten reference solutions were prepared,containing from 0.1 to 100 g of n-octane dissolved in 1 liter ofdodecane. The concentration of octane in the reference solutions shouldbe in the same area as the range of the volatiles in the samples to beanalysed. 4 μl of each solution was injected into a 20 ml injectionflask, which was thermostated to 120° C. and analysed. A calibrationfactor Rf for the area under the n-octane peak, A, vs. the amount ofn-octane in the solution in μg, C, was thus obtained as Rf=C/A.

The analysis was conducted as follows: The polymer sample (about 2grams) was placed in the 20 ml injection flask, which was thermostatedto 120° C. and kept at that temperature for one hour. A gas sample fromthe injection flask was then injected into the GC. Before the analysis,a blind run was conducted, where an injection from an empty flask wasmade. The hydrocarbon emission E was then calculated as follows:E=AT·Rf/W·1000000wherein

-   E is the hydrocarbon emission as μg volatile compounds per gram of    sample,-   AT is the total area under the sample peaks in area counts,-   Rf is the calibration factor for n-octane in μg per area count, and-   W is the weight of the sample in grams.    Rheology

Rheological parameters such as Shear Thinning index SHI and Viscosityare determined by using a rheometer, preferably an Anton Pear PhysicaMCR 300 Rheometer on compression moulded samples under nitrogenatmosphere at 190° C. using 25 mm diameter plates and plate and plategeometry with a 1.8 mm gap according to ASTM 1440-95. The oscillatoryshear experiments were done within the linear viscosity range of strainat frequencies from 0.05 to 300 rad/s (ISO 6721-1). Five measurementpoints per decade were made. The method is described in detail in WO00/22040.

The values of storage modulus (G′), loss modulus (G″) complex modulus(G*) and complex viscosity (η*) were obtained as a function of frequency(ω). η₁₀₀ is used as abbreviation for the complex viscosity at thefrequency of 100 rad/s.

Shear thinning index (SHI), which correlates with MWD and is independentof Mw, was calculated according to Heino (“Rheological characterizationof polyethylene fractions” Heino, E. L., Lehtinen, A., Tanner J.,Seppäiä, J., Neste Oy, Porvoo, Finland, Theor. Appl. Rheol., Proc. Int.Congr. Rheol, 11th (1992), 1, 360-362, and “The influence of molecularstructure on some rheological properties of polyethylene”, Heino, E. L.,Borealis Polymers Oy, Porvoo, Finland, Annual Transactions of the NordicRheology Society, 1995.).

SHI value is obtained by calculating the complex viscosities at givenvalues of complex modulus and calculating the ratio of the twoviscosities. For example, using the values of complex modulus of 2.7 kPaand 210 kPa, then η*(2.7 kPa) and η*(210 kPa) are obtained at a constantvalue of complex modulus of 2.7 kPa and 210 kPa, respectively. The shearthinning index SHI_(2.7/210) is then defined as the ratio of the twoviscosities η*(2.7 kPa) and η*(210 kPa), i.e. η(2.7)/η(210).

It is not always practical to measure the complex viscosity at a lowvalue of the frequency directly. The value can be extrapolated byconducting the measurements down to the frequency of 0.126 rad/s,drawing the plot of complex viscosity vs. frequency in a logarithmicscale, drawing a best-fitting line through the five points correspondingto the lowest values of frequency and reading the viscosity value fromthis line.

Example 1

Preparation of the Catalyst

Complex Preparation:

The catalyst complex used in the polymerisation example was bis(n-butylcyclopentadienyl)hafnium dibenzyl, (n-BuCp)₂Hf(CH₂Ph)₂, and it wasprepared according to “Catalyst Preparation Example 2” of WO2005/002744, starting from bis(n-butylcyclopentadienyl)hafniumdichloride (supplied by Witco).

Activated Catalyst System:

The catalyst was prepared according to Example 4 of WO-A-03/051934,except that 98.4 mg of bis(n-butyl cyclopentadienyl)hafnium dibenzylprepared as above was used as the metallocene compound instead of 80.3mg bis(n-butyl cyclopentadienyl)hafnium dichloride.

Multi-Stage Polymerisation

A loop reactor having a volume of 50 dm³ was operated as aprepolymerisation reactor at 80° C. and 63 bar pressure. Into thereactor were introduced 50 kg/h of propane diluent, 2 kg/h ethylene, 1.8g/h of hydrogen and 33 g/h of 1-butene. In addition, polymerisationcatalyst prepared according to the description above was introduced intothe reactor at a rate of 15 g/h.

The slurry was continuously withdrawn and directed into a subsequentloop reactor having a volume of 500 dm³, operated at 85° C. and 58 barpressure. Into the reactor were additionally introduced 97 kg/h ofpropane diluent, 42 kg/h ethylene and 13 g/h of a gas mixture containing25 vol-% of hydrogen in nitrogen. No additional comonomer was introducedinto the reactor. The polymerisation rate was 34 kg/h and the conditionsin the reactor as shown in Table 1.

The polymer slurry was withdrawn from the loop reactor and transferredinto a flash vessel operated at 3 bar pressure and 70° C. temperaturewhere the hydrocarbons were substantially removed from the polymer. Thepolymer was then introduced into a gas phase reactor operated at atemperature of 80° C. and a pressure of 20 bar. In addition 82 kg/hethylene, 1.3 kg/h 1-hexene and 7 g/h hydrogen was introduced into thereactor. The conditions are shown in Table 1.

The resulting polymer was purged with nitrogen (about 50 kg/h) for onehour, stabilised with 3000 ppm of Irganox B225 and 1500 ppm Ca-stearateand then extruded to pellets in a counter-rotating twin screw extruderCIM90P (manufactured by Japan Steel Works) so that the throughput was220 kg/h and the screw speed was 349 RPM.

Example 2

The procedure of Example 1 was repeated except that the operatingconditions were slightly changed. The data is shown in Table 1.

Comparative Example

Into a 50 dm³ loop reactor operated at 60° C. temperature and 63 barpressure as a prepolymerisation reactor were introduced ethylene (1.2kg/h), propane diluent, hydrogen and a polymerisation catalyst. Thesolid catalyst component was a commercially available product producedand sold by Engelhard Corporation in Pasadena, USA under a trade name ofLynx 200 (now supplied by BASF). The solid component was used togetherwith triethylaluminium cocatalyst so that the molar ratio of AIM wasfrom 30 to 100. The resulting ethylene homopolymer had an MFR₅ of 3.5g/10 min.

The slurry from the loop reactor was introduced into the second loopreactor having 500 dm³ volume operated at 85° C. and 57 bar whereadditional ethylene, propane and hydrogen were introduced. No comonomerwas introduced into the loop reactor. The resulting slurry was withdrawnfrom the reactor into a flash vessel where the polymer was separatedfrom the major fraction of the hydrocarbons at 70° C. and 3 bar. Thepolymer was directed into the gas phase reactor operated at 85° C. and20 bar where additional ethylene, 1-butene comonomer and hydrogen wereintroduced. The final polymer was mixed with the additives and extruded.Data is shown in Table 1.

TABLE 1 Experimental conditions and data Example 1 2 C.E.Prepolymerisation reactor Ethylene feed, kg/h 2.0 2.0 N.D Butene feed,g/h 33 33 N.D Hydrogen feed, g/h 1.8 1.5 N.D Catalyst feed, g/h 15 15N.D Loop reactor H₂/C₂, mol/kmol 0.18 0.17 947 Ethylene content, mol-%11.9 12.7 5.6 Production rate, kg/h 33 33 22 Polymer MFR₂, g/10 min 1113 442 Polymer Mw 68400 68000 Polymer density, kg/m³ 961 961 975 Gasphase reactor H₂/C₂, mol/kmol 0.10 0.08 48 C₄/C₂, mol/kmol 0.0 0.0 218C₆/C₂, mol/kmol 3.2 2.4 0 Ethylene content, mol-% 54 57 18 Productionrate, kg/h 35 37 28 Split, Prepol/LMW/HMW, %/%/% 3/48/49 3/48/49 2/44/54Calculated density, kg/m³ 927 928 912 Extruder Throughput, kg/h 220 220221 SEI, kWh/ton 265 270 280 Melt temperature, ° C. 225 230 226 Finalpolymer Polymer MFR₅, g/10 min 0.65 0.98 0.91 Polymer MFR₂₁, g/10 min7.4 9.4 25 Polymer density, kg/m³ 944.8 944.3 941.0 SHI_(2.7/210) N.D.7.6 33 η_(2.7), Pas N.D 28700 52000 Mw 181000 193000 N.D Mn 37500 38800N.D Mw/Mn 4.8 5 N.D Volatiles, mg/kg 4 5 120 Homogeneity, ISO 18553 5.6N.D. N.D. Gels in class 1, n 0 0 Gels in class 2, n 0 0 N.D = Notdetermined

The invention claimed is:
 1. A polymer composition comprising a multimodal copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms, the multimodal copolymer having a density from 924 to 960 kg/m³, an MFR₅ of from 0.4 to 6.0 g/10 min, an SHI_(2.7/210) from 1 to 30, an MFR₂₁/MFR₅ of between about 9.6 and about 11.4, and the composition has a level of volatile compounds of at most 100 ppm by weight,
 2. The polymer composition according to claim 1 having a melt index MFR₅ , of from 0.5 to 2.0 g/10 min and a melt index MFR₂ of from 0.1 to 2.0 g/10 min.
 3. The polymer composition according to claim 1, wherein the multimodal copolymer is a copolymer of ethylene and one or more alpha-olefins having from 6 to 8 carbon atoms.
 4. The polymer composition according to claim 1, wherein the multimodal copolymer comprises: (A) from 30 to 70% by weight, based on the combined amount of components (A) and (B), of a low molecular weight ethylene polymer selected from ethylene homopolymer and a copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms and having a weight average molecular weight of from 5000 to 100000 g/mol and a density of from 945 to 977 kg/m³; and (B) from 30 to 70% by weight, based on the combined amount of components (A) and (B), of a high molecular weight copolymer of ethylene and one or more alpha-olefins having from 4 to 10 carbon atoms and having a weight average molecular weight of from 100000 to 1000000 g/lmol and a density of from 890 to 935 kg,/m³.
 5. The polymer composition according to claim 4, wherein the low molecular weight ethylene polymer (A) is an ethylene homopolymer.
 6. The polymer composition according to claim 4, wherein the high molecular weight copolymer is a copolymer of ethylene and one or more alpha-olefins having from 6 to 8 carbon atoms.
 7. A process for producing the polymer composition according to claim 4, comprising the steps of: (i) polymerising, in a first polymerisation step in a first polymerisation zone, in the presence of a single site polymerisation catalyst, ethylene, hydrogen and optionally one or more alpha-olefins having 4 to 10 carbon atoms to form the low molecular weight component (A) having a weight average molecular weight of 5000 to 100000 g/mol and a density of from 945 to 977 kg/m³; (ii) polymerising, in a second polymerisation step in a second polymerisation zone, in the presence of a single site polymerisation catalyst, ethylene, one or more alpha-olefins having 4 to 10 carbon atoms and optionally hydrogen to form the high molecular weight component (B) having a weight average molecular weight of from 100000 to 1000000 g/mol and a density of from 890 to 935 kg/m³; wherein the first polymerisation step and the second polymerisation step may be conducted in any order and the subsequent step is conducted in the presence of the polymer produced in the prior step and the components (A) and (B) are present in the amounts of 30 to 70 and 70 to 30%, respectively, based on the combined amounts of components (A) and (B).
 8. The process according to claim 7, wherein said catalyst is a single site catalyst comprising a metallocene compound and an activator compound.
 9. The process according to claim 8, wherein the activator compound is an alumoxane, preferably a methylalumoxane.
 10. The process according to claim 8, wherein the metallocene compound is selected from the group consisting of [ethylenebis(3,7-di(tri-isopropylsiloxy)inden-1-yl)]zirconium dichloride, [ethylenebis(4,7-di(tri-isopropylsiloxy)inden-1-yl)]zirconium dichloride, [ethylenebis(5-tert- butyldimethylsiloxy)inden-1-yl)]zirconium dichloride, bis(5-tert-butyldimethylsiloxy)inden-1-yl)]zirconium dichloride, [dimethylsilylenebis( 5-tert -butyldimethylsiloxy)inden -1-yl)]zirconium dichloride, (N-tert-butylamido)(dimethyl)(η⁵-inden-4 -yloxy)silanetitanium dichloride, [ethylenebis(2- (tert-butydimethylsiloxy)inden-1-yl)]zirconium dichloride, bis(n- butylcyclopentadienyl)hafnium dichloride, bis(n - butylcyclopentadienyl) dibenzylhafnium, dimethylsilylenebis(n -butylcyclopentadienyl)hafnium dichloride, bis[1,2,4-tri(ethyl)cyclopentadienyl]hafnium dichloride, bis(4,5,6,7-tetrahydroindenyl)zirconium dichloride, bis(4,5,6,7-tetrahydroindenyl)hafnium dichloride, ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride and dimethylsilylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride.
 11. A pipe comprising the composition according to claim
 1. 12. A pipe according to claim 11 comprising from 85 to 100% of said polymer composition.
 13. The polymer composition according to claim 2, having a melt index MFR₅ of from 0.6 to 1.4 g/10 min.
 14. The polymer composition according to claim 2, having a melt index MFR₂ of from 0.2 to 1.0 g/10 min.
 15. The polymer composition according to claim 2, having a melt index MFR₂ of from 0.2 to 0.45 g/10 min. 